Porous material and method for preparation thereof, and honeycomb structure

ABSTRACT

A porous material  1  wherein silicon carbide particles  2  as an aggregate are bonded with one another via silicon nitride  3  as a binder in such a state that pores  5  are present between the silicon carbide particles  2,  wherein no columnar silicon nitride (silicon nitride whisker) is formed on the surface of the silicon nitride  3  within each pore  5,  or that, even when silicon nitride whiskers are inevitably formed there, the number of the columnar silicon nitride having a thickness of more than 2 μm and an aspect ratio of less than 10 is greater than that of the columnar silicon nitride having a thickness of 2 μm or less or an aspect ratio of 10 or more. The present invention provides a porous material superior in heat resistance and gas permeability, a method for production thereof, and a honeycomb structure.

TECHNICAL FIELD

The present invention relates to a porous material, a method forproduction thereof, and a honeycomb structure. More particularly, thepresent invention relates to a porous material superior in heatresistance and fluid permeability, a method for production thereof, anda honeycomb structure formed by using the porous material.

BACKGROUND ART

Honeycomb structures are in use, for example, in filters for capture offine particles present in exhaust gas, particularly in diesel exhaustgas [hereinafter, the filter for capture of fine particles present indiesel exhaust gas is referred to as “DPF” (diesel particulate filter)],as well as in catalyst carriers used in internal combustion engine,boiler, chemical reactor, fuel cell reformer, etc. As a filter forcapture of fine particles, used in a high-temperature, corrosive gasatmosphere, such as DPF, there is used suitably a porous honeycombstructure made of a ceramic. As a material for such a honeycombstructure used in a high-temperature, corrosive gas atmosphere, there isused suitably a silicon-silicon carbide porous material superior in heatresistance and chemical stability, wherein silicon carbide is bonded viasilicon used as a binder, because, with such a porous material, it iseasy to control the porosity, pore diameter, etc. of the resultinghoneycomb structure. Meanwhile, silicon nitride is a preferred materialfrom the standpoint of thermal shock resistance because it hashigh-temperature resistance and low thermal expansion coefficient.Hence, there was proposed a heat-resistance improved honeycomb structureby using a silicon-nitride-silicon carbide porous material whereinsilicon carbide is bonded via silicon nitride (instead of silicon) usedas a binder (see, for example, JP-A-H04-94736). In the conventionalmethod, however, there was a problem in that silicon nitride whiskersare formed in the pores of porous material, reducing the gaspermeability of porous material.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the above problem andprovides a porous material superior in heat resistance and gaspermeability, a method for production thereof, and a honeycombstructure.

According to the present invention, there are provided the followingporous material, method for production thereof, and honeycomb structure.

-   [1] A porous material wherein silicon carbide particles as an    aggregate are bonded with one another via silicon nitride as a    binder in such a state that pores are present between the silicon    carbide particles, wherein

no columnar silicon nitride (silicon nitride whisker) is formed on thesurface of the silicon nitride within each pore, or that,

even when columnar silicon nitride is inevitably formed there, thenumber of the columnar silicon nitride having a thickness of more than 2μm and an aspect ratio of less than 10 is greater than that of thecolumnar silicon nitride having a thickness of 2 μm or less or an aspectratio of 10 or more (hereinafter, this invention may be referred to as“first invention”).

-   [2] A porous material wherein silicon carbide particles as an    aggregate are bonded with one another via silicon nitride as a    binder in such a state that pores are present between the silicon    carbide particles, wherein the pores have a specific surface area of    1 m²/g or less (hereinafter, this invention may be referred to as    “second invention”).-   [3] A porous material according to [1] or [2], wherein an open    porosity is 40 to 75%.-   [4] A porous material according to any of [1] to [3], wherein the    pores have an average pore diameter of 5 to 50 μm.-   [5] A porous material according to any of [1] to [4], which has a    heat resistance temperature of 1,200° C. or more.-   [6] A porous material according to any of [1] to [5], which has a    gas permeability coefficient of 1 μm² or more.-   [7] A method for producing a porous material set forth in any of [1]    to [6], wherein the method comprises the steps of:

mixing at least silica, silicon nitride and a pore former;

firing the resulting mixture at 1,400 to 1,500° C. in an inert gasatmosphere or reduced-pressure atmosphere where the oxygen partialpressure is 10 Pa or less to prepare a silicon-silicon carbide porousmaterial; and

nitriding and firing the silicon-silicon carbide porous material at1,200 to 1,800° C. in a nitrogen atmosphere.

-   [8] A method for producing a porous material according to [7],    wherein, after preparing the silicon-silicon carbide porous    material, the atmosphere used therein is changed to a nitrogen    atmosphere without lowering the temperature to room temperature and    keeping the temperature at 1,200° C. or more, and nitriding and    firing the silicon nitride-silicon carbide porous material at 1,200    to 1,800° C. in the nitrogen atmosphere is conducted.-   [9] A method for producing a porous material according to [7],    wherein, after preparing the silicon-silicon carbide porous    material, nitriding and firing the silicon-silicon carbide porous    material at 1,200 to 1,800° C. is conducted in a nitrogen atmosphere    containing 0.1% by volume or more of hydrogen.-   [10] A method for producing a porous material according to [7],    wherein, after the preparation of the silicon-silicon carbide porous    material, the atmosphere is changed to a nitrogen atmosphere    containing 0.1% by volume or more of hydrogen (a hydrogen-containing    nitrogen atmosphere) without lowering the temperature to room    temperature and keeping the temperature at 1,200° C. or more, and    nitriding and firing the silicon-silicon carbide porous material at    1,200 to 1,800° C. in the hydrogen-containing nitrogen atmosphere is    conducted.-   [11] A honeycomb structure constituted by a porous material set    forth in any of [1] to [6].

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partly enlarged sectional view showing an embodiment of theporous material of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

First, description is made on the first invention. FIG. 1 is a partlyenlarged sectional view showing an embodiment of the porous material ofthe present invention.

As shown in FIG. 1, in the porous material 1 of the present embodiment,silicon carbide (SiC) particles 2 as an aggregate are bonded with oneanother via silicon nitride (Si₃N₄) 3 as a binder in such a state thatpores 5 are present between the silicon carbide particles 2. As shown inFIG. 1, it is particularly preferred that there is present, on thesurface of the silicon nitride 3 within each pore 5, no columnar siliconnitride (hereinafter, this may be referred to as silicon nitridewhisker) having a thickness of smaller than 2 μm and an aspect ratio oflarger than 10. Thus, no silicon nitride whisker is present in each pore5 and, even when silicon nitride whiskers are inevitably present there,the number of the columnar silicon nitride having a thickness of morethan 2 μm and an aspect ratio of less than 10 is greater than that ofthe columnar silicon nitride having a thickness of 2 μm or less or anaspect ratio of 10 or more; therefore, it does no happen that part ofthe pores are blocked by silicon nitride whiskers, and the porousmaterial of the present embodiment is superior in gas permeability. Thatis, in the porous material of the present embodiment, it can be saidthat substantially no silicon nitride whisker is present, from thestandpoint of gas permeability. This feature appears strikingly whenthere is present no columnar silicon nitride. Further, in the porousmaterial 1, silicon carbide particles 2 as an aggregate are bonded withone another via silicon nitride 3 as a binder; therefore, the porousmaterial 1 is superior in heat resistance.

The thickness of the columnar silicon nitride refers to a width ofcolumnar silicon nitride in a direction perpendicular to thelongitudinal direction of columnar silicon nitride, in a SEM photographof a fracture surface or a surface of a porous material at amagnification of 5,000 times. In determination of this thickness, thethickness is measured at four points of equal intervals in longitudinal,and the average of the measurements is taken as the thickness of onecolumnar silicon nitride. The aspect ratio of the columnar siliconnitride is a value obtained by dividing the length of columnar siliconnitride in its length direction, in a SEM photograph of a fracturesurface or a surface of porous material at a magnification of 5,000times, by the thickness of the columnar silicon nitride. The length ofthe columnar silicon nitride in its longitudinal direction refers to alength of columnar silicon nitride from one end of columnar siliconnitride connecting to the surface of silicon nitride 3 forming pores 5,to the other end of columnar silicon nitride. When the columnar siliconnitride is branched, its length in the longitudinal direction refers toa length of columnar silicon nitride from one end of columnar siliconnitride connecting to the surface of silicon nitride 3 forming pores 5,to each branch end of columnar silicon nitride. The sectional shape ofthe columnar silicon nitride may be any of circle, oval, oblong shape,polygon (e.g. triangle), modification thereof, and indeterminate shape.

The open porosity of the porous material of the present embodiment ispreferably 40 to 75%, more preferably 50 to 75%, particularly preferably55 to 75%. When the open porosity is less than 40%, the porous materialmay cause a large pressure loss and may have a low gas permeabilitycoefficient. When the open porosity is more than 75%, the porousmaterial may have a low strength.

The average pore diameter of the porous material of the presentembodiment is preferably 5 to 50 μm, more preferably 10 to 40 μm,particularly preferably 15 to 30 μm. When the average pore diameter isless than 5 μm, the porous material may cause a large pressure loss andmay have a low gas permeability coefficient. When the average porediameter is more than 50 μm, the porous material may have a lowstrength.

The gas permeability coefficient of the porous material of the presentembodiment is preferably 1 μm² or more. When the gas permeabilitycoefficient is less than 1 μm², gas permeation is difficult when theporous material is used, for example, as a DPF, and the system may havea high load.

The heat resistance temperature of the porous material of the presentembodiment is preferably 1,200° C. or higher. When the temperature islower than 1,200° C, there may arise a problem in heat resistancedepending upon the use conditions of the porous material. The heatresistance temperature is the highest temperature at which the porousmaterial shows no change in microstructure and properties, andspecifically is the highest temperature at which, when the porousmaterial is subjected to an oxidation treatment for 24 hours in a highoxygen partial pressure (such as in the air), the mass increase of theporous material does no exceed 5% by mass.

In the porous material of the present embodiment, the specific surfacearea of pores is preferably 1 m²/g or less, more preferably 0.2 to 0.9m²/g, further preferably 0.3 to 0.9 m²/g, particularly preferably 0.3 to0.5 m²/g, most preferably 0.3 to 0.4 m²/g. When the specific surfacearea is larger than 1 m²/g, the pore-inside surface shape becomes morecomplicated; the gas passages become more curved or enlarged, or haveincreased narrow portions; and gas permeability may be impaired.

An embodiment of the honeycomb structure of the present inventionconstituted by the porous material of the present embodiment is ahoneycomb structure having a plurality of cells (which become passagesof fluid) surrounded and formed by the partition walls made of theporous material of the present embodiment.

Since the partition walls are made of the porous material of the presentembodiment, the honeycomb structure of the present embodiment issuperior in heat resistance and gas permeability coefficient.

In the honeycomb structure of the present embodiment, there is noparticular restriction as to the cell sectional shape, the cellsectional diameter, the cell density, the partition wall thickness,etc.; and they can be determined appropriately depending upon theapplication of the honeycomb structure.

Next, description is made on an embodiment of the porous material of thesecond invention. As shown in FIG. 1, in the porous material 1 of thepresent embodiment, as in the porous material of the first invention,silicon carbide (SiC) particles 2 as an aggregate are bonded with oneanother via silicon nitride (Si₃N₄) 3 as a binder in such a state thatpores 5 are present between the silicon carbide particles 2. In theporous material 1 of the present embodiment, the pores 5 of the porousmaterial 1 have a specific surface area of 1 m²/g or less. The specificsurface area is preferably 0.2 to 0.9 m²/g, more preferably 0.3 to 0.9m²/g, particularly preferably 0.3 to 0.5 m²/g, most preferably 0.3 to0.4 m²/g. When the specific surface area is larger than 1 m²/g, thepore-inside surface shape becomes more complicated; the gas passagesbecome more curved or enlarged, or have increased narrow portions; andgas permeability is impaired significantly.

Here, “specific surface area” indicates a surface area per unit mass andis determined, for example, by determining, based on the BET theory, thenumber (N) of molecules necessary for the mono-molecular layer of thegas physically adsorbed on a sample surface to cover the sample surface,multiplying the molecular number (N) by the molecular sectional area ofthe adsorbed gas to calculate the surface area of the sample, anddividing the surface area of the sample by the mass of the sample.

In the porous material of the present embodiment, the open porosity, theaverage pore diameter, the heat resistance temperature and the gaspermeability coefficient are preferably the same as in the porousmaterial of the first invention. Thereby can be obtained the sameeffects as in the porous material of the first invention.

Another embodiment of the honeycomb structure of the present inventionconstituted by the porous material of the present embodiment can havethe same constitution as that of the honeycomb structure constituted bythe porous material of the first invention, whereby the same effects canbe obtained.

Below is described an embodiment of the method for producing the porousmaterial of the first invention or the porous material of the secondinvention.

Silicon, silicon carbide and, as necessary, a pore former are mixed toprepare a raw material for firing. The use amounts of silicon andsilicon carbide are preferably 10 to 40% mass and 60 to 90% by mass,respectively, based on the total of the two. The use amount of the poreformer is preferably 5 to 40% by mass based on the total of silicon andsilicon carbide. It is preferred to add, to the raw material for firing,a compound containing at least one element selected from the groupconsisting of group 2A, group 3A, group 3B including lanthanoid elementsand group 4B. This compound is added preferably in an amount of 0.1 to10% by mass based on the total of silicon and silicon carbide. Informing the raw material for firing, there may be added as necessaryforming aids such as organic binder, surfactant, water and the like.

Silicon is preferably a metallic silicon powder having an averageparticle diameter of 1 to 20 μm. Silicon carbide is preferably a siliconcarbide powder having an average particle diameter of 10 to 50 μm.

The element selected from the group consisting of group 2A, group 3A,group 3B including lanthanoid elements and group 4B is added ordinarilyin an oxide form. However, it may be added in a form such as carbonate,nitrate, fluoride, nitride, carbide or the like. As specific examples ofthe oxide, there can be mentioned MgO, CaO, SrO, BaO, Al₂O₃, Y₂O₃, CeO₂,Sm₂O₃, Er₂O₃, Yb₂O₃, TiO₂, ZrO₂ and HfO₂. There may also be used theircarbonates, nitrates, fluorides, nitrides and carbides.

As the pore former, there can be used organic substances such as starch,cellulose, foamed resin and the like.

Next, the raw material for firing obtained (this may be a formedmaterial obtained by kneading the raw material for firing to prepareclay for forming, and forming the clay into an intended shape byextrusion or the like) is fired in an inert gas or reduced pressureatmosphere wherein the oxygen partial pressure is 10 Pa or less, toprepare a silicon-silicon carbide porous material. Here, thesilicon-silicon carbide porous material is a porous material whereinsilicon carbide particles as an aggregate are bonded with one anothervia silicon as a binder in such a state that pores are present betweenthe silicon carbide particles. The temperature of the firing ispreferably 1,400 to 1,500° C. As the inert gas, argon is preferred.

After the preparation of the silicon-silicon carbide porous material,the atmosphere used therein is changed to a nitrogen atmosphere or to anitrogen atmosphere containing 0.1% by volume or more of hydrogen (ahydrogen-containing atmosphere) without lowering the temperature used inthe above firing, of 1,400 to 1,500° C. to room temperature and keepingthe temperature at 1,200° C. or more, and firing and nitriding areconducted at 1,200 to 1,800° C., whereby a silicon nitride-siliconcarbide porous material (which is a porous material of the presentembodiment) can be obtained. Here, the content of hydrogen in thehydrogen-containing nitrogen atmosphere is 0.1% by volume or more of thetotal atmosphere, preferably 1 to 30% by volume, more preferably 4 to10% by volume. In the nitriding reaction, the silicon in thesilicon-silicon carbide porous material is nitrided. By thus conducting,after firing of the silicon-silicon carbide porous material, nitridingand firing without lowering the atmospheric temperature to roomtemperature, the energy required for the nitriding and firing can bemade smaller and the firing time can be shortened. As a result, the loadto environment and the cost of production can be reduced. Thus, it ispreferred not to lower the atmospheric temperature to room temperatureafter the preparation of silicon-silicon carbide porous material;however, it is possible to once lower the atmospheric temperature toroom temperature and then increase the temperature to conduct nitridingand firing.

In conducting the above nitriding, columnar silicon nitride may beformed. The mechanism of formation of the columnar silicon nitride isconsidered to be such that the oxide present on the surface of metallicsilicon vaporizes as a SiO gas and reacts with nitrogen gas, and thereaction product deposits as columnar silicon nitride.

Here, the silicon nitride-silicon carbide porous material is a porousmaterial wherein silicon carbide particles as an aggregate are bondedwith one another via silicon nitride as a binder in such a state thatpores are present between the silicon carbide particles.

As described above, nitriding and firing are conducted at 1,200 to1,800° C. and, therefore, the once-formed columnar silicon nitride meltsand grows again as a large crystal. As a result, the columnar siliconnitride decreases or disappears in the porous material obtained; and nocolumnar silicon nitride (silicon nitride whisker) having a thickness ofless than 2 μm and an aspect ratio of larger than 10 is present on thesurface of the silicon nitride within each pore, or, even when siliconnitride whiskers are inevitably present there, the number of thecolumnar silicon nitride having a thickness of more than 2 μm and anaspect ratio of less than 10 is greater than that of the columnarsilicon nitride having a thickness of 2 μm or less or an aspect ratio of10 or more. In this connection, the specific surface area of pores canbe made 1 m²/g or less. When the firing is conducted at a temperaturehigher than 1,800° C. in a nitrogen atmosphere (nitrogen: 100% byvolume) at normal pressure (atmospheric pressure), silicon nitridebegins to decompose; therefore, firing under applied pressure isnecessary to prevent such decomposition. Hence, firing at a temperaturehigher than 1,800° C. incurs a problem of higher facility cost. When thefiring is conducted at a temperature lower than 1,200° C., formation ofsilicon nitride is insufficient and silicon remains in a large amount.Also, part of the porous material may not be covered with siliconnitride and silicon may be exposed. As a result, the porous materialformed is low in heat resistance.

The compound containing at least one element selected from the groupconsisting of group 2A, group 3A, group 3B including lanthanoid elementsand group 4B, which is added to the raw material for firing, exhibits afunction as an aid capable of forming a liquid phase at hightemperatures. Therefore, when the compound as the aid is added to theraw material for firing and there are conducted nitriding of silicon andfiring at 1,200 to 1,800° C., the columnar silicon nitride formed bysilicon nitriding is heat-treated together with the compound as the aidat high temperatures of 1,200 to 1,800° C.; as a result, the cylindricalparticles of silicon nitride (Si₃N₄) dissolves in the compound (aid)which has become a liquid phase owing to high temperatures, the columnarsilicon nitride disappears, and there takes place deposition of largecylindrical particles. In this way, there can be obtained a porousmaterial of the present embodiment which is a silicon nitride-siliconcarbide porous material containing silicon nitride constituted mainly byβ-Si₃N₄ large cylindrical particles.

When the addition amount of the compound as the aid exceeds 10% by massbased on the total of silicon and silicon carbide, the porous materialobtained may be poor in high-temperature properties, for example, heatresistance. Depending upon the kind of the aid used, the porous materialobtained may be high in thermal expansion coefficient. When the additionamount of the compound as an aid is less than 0.1% by mass, the functionof the aid may not be exhibited sufficiently.

Meanwhile, when nitriding is conducted in a nitrogen atmospherecontaining at least 0.1% by volume of hydrogen at 1,200 to 1,800° C., aporous material of the present embodiment (which is a siliconnitride-silicon carbide porous material) can be obtained withoutconducting firing at high temperatures of 1,200 to 1,800° C. in order todissolve columnar silicon nitride. With a high nitriding temperature,the energy required for temperature elevation is large, the load toenvironment is large, and the cost of preparation is high; therefore,the high nitriding temperature is not preferred. When nitriding isconducted in a hydrogen-containing nitrogen atmosphere, the sameintended porous material can be obtained even at 1,500° C. or below.

When, as described above, change of atmosphere is made to a nitrogenatmosphere or a nitrogen atmosphere containing at least 0.1% by volumeof hydrogen, keeping the temperature at 1,200° C. or more withoutlowering the temperature used in firing, of 1,400 to 1,500° C. to roomtemperature and in this state, nitriding and firing are conducted at1,200 to 1,800° C., the temperature at which the change of atmosphere ismade, is preferred to be not lower than the above-mentioned firingtemperature of 1,400 to 1,500° C. Thereby, there can be obtained aporous material wherein there is present, on the surface of the siliconnitride within each pore, no columnar silicon nitride (silicon nitridewhisker) having a thickness of less than 2 μm and an aspect ratio ofmore than 10, or, even when silicon nitride whiskers are inevitablypresent there, the number of the columnar silicon nitride having athickness of more than 2 μm and an aspect ratio of less than 10 isgreater than that of the columnar silicon nitride having a thickness of2 μm or less or an aspect ratio of 10 or more.

When the silicon of the silicon-silicon carbide porous material isnitrided, it is preferred that the nitriding ratio of silicon (which isa value indicating the proportion of metallic silicon converted intosilicon nitride and can be calculated from an X-ray diffractionintensity ratio of metallic silicon and silicon nitride) is 90% or more.When the nitriding ratio of silicon is less than 90%, the resultingporous material is high in thermal expansion coefficient, and, ifmetallic silicon remains in a large amount, there may be deteriorationin high-temperature properties such as heat resistance andhigh-temperature strength.

Further, since the silicon in silicon-silicon carbide porous material isnitrided, the silicon can be converted into silicon nitride in a statethat silicon carbide is bonded via silicon, and a high strength can beretained.

The raw materials for firing are mixed and kneaded to prepare clay forforming; the clay is formed into an intended shape by extrusion or thelike to prepare a formed material; in this case, the formed material isallowed to have a honeycomb structure; and the formed material ofhoneycomb structure is fired, nitrided, and fired at high temperatures;thereby can be obtained a honeycomb structure of the present invention.

EXAMPLES

The present invention is described more specifically below by way ofExamples. However, the present invention is in no way restricted tothese Examples.

Example 1

To 100% by mass of the total of 80% by mass of silicon carbide (SiC)having an average particle diameter of 47 μm and 20% by mass of ametallic silicon (Si) powder having an average particle diameter of 5 μmwere added, as superaddition, 1% by mass of strontium carbonate whichwas a compound as the aid (hereinafter, this compound may be referred toas “aid”), 10% by mass of starch as a pore former, 8% by mass of methylcellulose as a binder, 1% by mass of a surfactant and 19% by mass ofwater. They were mixed to prepare a raw material before firing. The rawmaterial was kneaded by a kneader and subjected to extrusion, to preparea formed material of honeycomb structure. The formed material ofhoneycomb structure was held in the air at 500° C. to remove the binder.Then, the resulting material was fired (before-nitriding firing offormed material) at 1.3 Pa in an argon atmosphere at 1,450° C. for 2hours, to obtain a silicon-silicon carbide porous material of honeycombstructure. After the preparation of the silicon-silicon carbide porousmaterial, the system temperature was lowered to room temperature and, atthe room temperature, the atmosphere was changed to a nitrogenatmosphere. The room temperature was increased to 1,450° C. and theabove-obtained silicon-silicon carbide porous material of honeycombstructure was maintained at normal pressure (atmospheric pressure) in anitrogen atmosphere at 1,450° C. for 4 hours to give rise to nitriding,after which the resulting material was maintained at 1,750° C. for 4hours (during-nitriding firing) to obtain a sintered material (ahoneycomb ceramic structure). The conditions used for preparation ofthis material are shown in Table 1. In Table 1, in the lateral column of“before-nitriding firing and during-nitriding firing”, “separatefirings” indicate that, after preparation of silicon-silicon carbideporous material by firing, the temperature used is decreased to roomtemperature and then nitriding and firing are conducted (firing isconducted two times), and “same firing” indicates that, afterpreparation of silicon-silicon carbide porous material by firing,nitriding and firing are conducted without temperature decrease (twofirings are conducted continuously). Also in Table 1, “temperature ofatmosphere change” indicates a temperature at which, in nitriding ofsilicon-silicon carbide porous material prepared, the atmosphere used inpreparation of the material is changed to a nitrogen atmosphere. Testpieces of 4 mm×3 mm×40 mm, 10 mm (diameter)×3 mm, etc. were cut out fromthe sintered material and measured for the following evaluation items.The results of measurements are shown in Table 3.

(Evaluation Items)

Open Porosity

Measured by an in-water weight method.

Average Pore Diameter

Measured using a mercury porosimeter.

Four-point Bending Strength

Measured using a cut-out test piece of 4 mm×3 mm×40 mm, according to JISR 1601.

Comparison of “Thickness” and “Aspect Ratio” of Columnar Silicon Nitride(Condition 1)

“Thickness” and “aspect ratio” of columnar silicon nitride were measuredas described below, and there was made a comparison between “the numberof columnar silicon nitride having a thickness of more than 2 μm and anaspect ratio of less than 10 (this number is arbitrarily expressed asA)” and the number of columnar silicon nitride having a thickness of 2μm or less and an aspect ratio of 10 or more (this number is arbitrarilyexpressed as B)”. A case when A was larger than B, was evaluated as“suitable”; and a case when A was smaller than B, was evaluated as“unsuitable”.

Thickness of Columnar Silicon Nitride (Columnar Thickness)

In a SEM photograph (a magnification of 5,000 times) of a fracturesurface or a surface of porous material, there was measured a width ofcolumnar silicon nitride in a direction perpendicular to itslongitudinal direction. This length was measured at four points inlongitudinal direction at equal intervals, and an average thereof wastaken as thickness of one columnar silicon nitride.

Aspect Ratio of Columnar Silicon Nitride (Aspect Ratio)

In a SEM photograph (a magnification of 5,000 times) of the fracturesurface or the surface of porous material, there were measured alongitudinal direction length of columnar silicon nitride and itsthickness; and the length was divided by the thickness. The valueobtained was taken as aspect ratio.

Specific Surface Area

Measured by a gas adsorption method using a volumetric method.

Mass Change After Treatment in the Air at 1,200° C. for 24 Hours (1,200°C.×24 hr treatment)

A porous material was maintained in the air at 1,200° C. for 24 hoursand the mass change thereof was measured. This mass change is a valueobtaining by subtracting the mass of the porous material after the abovetreatment of maintaining it at 1,200° C. for 24 hours, from the mass ofthe porous material before the treatment, diving the resultingdifference by the mass before the treatment, and multiplying theresulting quotient by 100.

Heat Resistance Temperature

The highest temperature at which mass change after a treatment in theair for 24 hours does not exceed 5%.

Gas Permeability Coefficient

Determined based on the following measurement and calculation accordingto the Darcy's law, in consideration of gas compressibility. From ahoneycomb ceramic structure was cut out a part of the partition wall.The cut-out part was processed into a plate having no surface unevennessto use it as a sample. This sample was sandwiched at the two flatsurface portions (two sides) by two cylindrical tubes so that the twotubes faced each other at the open ends and there occurred no gasleakage. In this case, the two cylindrical tubes were allowed to faceeach other at their open ends via the sample and the central axes of thetwo cylindrical tubes coincided with each other. Then, the air was fedinto one of the cylindrical tubes to allow the air to pass through thecylindrical tubes, with the flow amount of the air being controlled. Inthis case, the pressures at the upstream side and downstream side of thesample were measured and the gas permeability coefficient of the samplewas determined using the following expression (1). Incidentally, in thefollowing expression (1); K is a gas permeability coefficient (μm²); Qis an amount (m³/s) of gas which passed, measured at the sampledownstream side; T is a sample thickness (m); μ is a static viscositycoefficient (Pa·s) of gas which passed; D is a diameter (m) of sample atthe portion which gas passed; P1 is a gas pressure (Pa) at upstreamside; and P2 is a gas pressure (Pa) at downstream side.K=[8·μ·T·Q·P2]÷[π·D ²·(P1−P2²)]×10¹²   (1)

Example 2

After “obtaining a silicon-silicon carbide porous material of honeycombstructure” in Example 1, no temperature decrease was made and thetemperature of atmosphere was maintained at 1,400° C. and, in thisstate, the atmosphere was changed to a nitrogen atmosphere. Then, thesystem was maintained at normal pressure (atmospheric pressure) in anitrogen atmosphere at 1,450° C. for 4 hours to conduct a nitridingtreatment. Thereafter, the system was maintained at 1,750° C. for 4hours (during-nitriding firing) to obtain a sintered material (ahoneycomb ceramic structure). The preparation conditions are shown inTable 1. Other conditions were the same as in Example 1. In the samemanner as in Example 1, test pieces were cut out from the sinteredmaterial obtained and measured for the above-mentioned evaluation items.The results of measurements are shown in Table 3.

Example 3

A sintered material was prepared (the preparation conditions are shownin Table 1), in the same manner as in Example 1 except that no poreformer was added (in Example 1, starch was added as a pore former). Testpieces were cut out from the sintered material obtained and measured forthe above-mentioned evaluation items in the same manner as in Example 1.The results of measurements are shown in Table 3.

Example 4

A sintered material was prepared (the preparation conditions are shownin Table 1), in the same manner as in Example 1 except that no aid wasadded (in Example 1, strontium carbonate was added as the aid). Testpieces were cut out from the sintered material obtained and measured forthe above-mentioned evaluation items in the same manner as in Example 1.The results of measurements are shown in Table 3.

Example 5

“The resulting material was fired at 1.3 Pa in an argon atmosphere at1,450° C. for 2 hours, to obtain a silicon-silicon carbide porousmaterial of honeycomb structure” in Example 1 was changed to “theresulting material was fired at normal pressure (atmospheric pressure)in an argon atmosphere (oxygen partial pressure: 10 Pa) at 1,450° C. for2 hours to obtain a silicon-silicon carbide porous material of honeycombstructure”. Other conditions were the same as in Example 1, to prepare asintered material (the preparation conditions are shown in Table 1).Test pieces were cut out from the sintered material obtained andmeasured for the above-mentioned evaluation items in the same manner asin Example 1. The results of measurements are shown in Table 3.

Example 6

A sintered material was prepared (the preparation conditions are shownin Table 1), in the same manner as in Example 1 except that the averageparticle diameter of silicon carbide (SiC) was changed to 24 μm. Testpieces were cut out from the sintered material obtained and measured forthe above-mentioned evaluation items in the same manner as in Example 1.The results of measurements are shown in Table 3.

Example 7

A sintered material was prepared (the preparation conditions are shownin Table 1), in the same manner as in Example 1 except that the additionamount of starch (pore former) was changed to 30% by mass. Test pieceswere cut out from the sintered material obtained and measured for theabove-mentioned evaluation items in the same manner as in Example 1. Theresults of measurements are shown in Table 3.

Example 8

A sintered material was prepared (the preparation conditions are shownin Table 1), in the same manner as in Example 1 except that the additionamount of pore former was changed to 40% by mass. Test pieces were cutout from the sintered material obtained and measured for theabove-mentioned evaluation items in the same manner as in Example 1. Theresults of measurements are shown in Table 3.

Example 9

A sintered material was prepared (the preparation conditions are shownin Table 1), in the same manner as in Example 1 except that the massratio of silicon carbide and silicon was changed to 70:30, the ratio ofthe mass total of silicon carbide and silicon to the mass of thebefore-firing raw material total was made the same as in Example 1, andthe aid was added so that the amount ratio of aid and silicon became thesame as the amount ratio of aid and silicon in Example 1. Test pieceswere cut out from the sintered material obtained and measured for theabove-mentioned evaluation items in the same manner as in Example 1. Theresults of measurements are shown in Table 3.

Example 10

A sintered material was prepared (the preparation conditions are shownin Table 1), in the same manner as in Example 1 except that the averageparticle diameter of silicon carbide (SiC) was changed to 68 μm. Testpieces were cut out from the sintered material obtained and measured forthe above-mentioned evaluation items in the same manner as in Example 1.The results of measurements are shown in Table 3.

Example 11

A sintered material was prepared (the preparation conditions are shownin Table 1), in the same manner as in Example 1 except that the averageparticle diameter of silicon carbide (SiC) was changed to 12 μm. Testpieces were cut out from the sintered material obtained and measured forthe above-mentioned evaluation items in the same manner as in Example 1.The results of measurements are shown in Table 3.

Example 12

A sintered material was prepared (the preparation conditions are shownin Table 1), in the same manner as in Example 1 except that the additionamount of aid (strontium carbonate) was changed to 3% by mass and that“the silicon-silicon carbide porous material of honeycomb structure wasmaintained at normal pressure (atmospheric pressure) in a nitrogenatmosphere at 1,450° C. for 4 hours and further maintained at 1,750° C.for 4 hours (during-nitriding firing)” was changed to “thesilicon-silicon carbide porous material of honeycomb structure wasmaintained at normal pressure (atmospheric pressure) in a nitrogenatmosphere at 1,200° C. for 12 hours”. In the same manner as in Example1, test pieces were cut out from the sintered material obtained andmeasured for the above-mentioned evaluation items. The results ofmeasurements are shown in Table 3.

Example 13

A sintered material was prepared (the preparation conditions are shownin Table 1), in the same manner as in Example 1 except that “at 1.3 Pain an argon atmosphere” was changed to “at normal pressure (atmosphericpressure) in an argon atmosphere (oxygen partial pressure: 10 Pa)” andthat “the silicon-silicon carbide porous material of honeycomb structurewas maintained at normal pressure (atmospheric pressure) in a nitrogenatmosphere at 1,450° C. for 4 hours to give rise to nitriding, afterwhich the resulting material was maintained at 1,750° C. for 4 hours(during-nitriding firing) to obtain a sintered material (a honeycombceramic structure)” was changed to “the silicon-silicon carbide porousmaterial of honeycomb structure was maintained at normal pressure(atmospheric pressure) in a nitrogen atmosphere containing 5% by volumeof hydrogen at 1,450° C. for 4 hours to obtain a sintered material (ahoneycomb ceramic structure)”. In the same manner as in Example 1, testpieces were cut out from the sintered material obtained and measured forthe above-mentioned evaluation items. The results of measurements areshown in Table 3.

Example 14

A sintered material was prepared (the preparation conditions are shownin Table 1), in the same manner as in Example 1 except that “at 1.3 Pain an argon atmosphere” was changed to “at normal pressure (atmosphericpressure) in an argon atmosphere (oxygen partial pressure: 10 Pa)” andthat “After the preparation of the silicon-silicon carbide porousmaterial, the temperature was lowered to room temperature and, at thistemperature, the atmosphere was changed to a nitrogen atmosphere. Theroom temperature was increased to 1,450° C. and the above-obtainedsilicon-silicon carbide porous material of honeycomb structure was heldat normal pressure (atmospheric pressure) in a nitrogen atmosphere at1,450° C. for 4 hours to give rise to nitriding, after which theresulting material was held at 1,750° C. for 4 hours (during-nitridingfiring) to obtain a sintered material (a honeycomb ceramic structure)”was changed to “After the preparation of the silicon-silicon carbideporous material, the system atmosphere was changed to a nitrogenatmosphere containing 5% by volume of hydrogen, in a state that thesystem temperature was maintained at 1,450° C. without lowering it toroom temperature, and the silicon-silicon carbide porous material wasmaintained at 1,450° C. for 4 hours to obtain a sintered material (ahoneycomb ceramic structure)”. In the same manner as in Example 1, testpieces were cut out from the sintered material obtained and measured forthe above-mentioned evaluation items. The results of measurements areshown in Table 3.

Comparative Example 1

A formed material of honeycomb structure was prepared by extrusion, inthe same manner as in Example 1. The formed material was maintained inthe air at 500° C. to remove the binder contained therein and thenmaintained at normal pressure (atmospheric pressure) in a nitrogenatmosphere at 1,450° C. for 4 hours to conduct nitriding. Thereafter,the nitrided material was maintained at 1,750° C. for 4 hours to obtaina nitrided formed material of honeycomb structure. This material was notsintered and, when touched, collapsed. Therefore, cutting out of testpieces therefrom was impossible (the preparation conditions are shown inTable 2), and measurements for the above-mentioned evaluation items wereimpossible.

Comparative Example 2

A silicon-silicon carbide porous material of honeycomb structure wasobtained in the same manner as in Example 1 except that, in thebefore-nitriding firing of formed material, the formed material wasfired at normal pressure (atmospheric pressure) in an argon atmosphere(oxygen partial pressure: 100 Pa) at 1,450° C. for 2 hours. A sinteredmaterial was prepared from the silicon-silicon carbide porous materialin the same manner as in Example 1 (the preparation conditions are shownin Table 2). Test pieces were cut out from the sintered material andmeasured for the above-mentioned evaluation items in the same manner asin Example 1. The results of measurements are shown in Table 4.

Comparative Example 3

A sintered material was prepared in the same manner as in Example 1 (thepreparation conditions are shown in Table 2) in the same manner as inExample 1 except that the average particle diameter of silicon carbide(SiC) was changed to 24 μm and that “before-nitriding firing of formedmaterial” was changed to “The formed material was fired at normalpressure (atmospheric pressure) in an argon atmosphere (oxygen partialpressure: 100 Pa) at 1,450° C. for 2 hours to obtain a silicon-siliconcarbide porous material of honeycomb structure”. Test pieces were cutout from the sintered material and measured for the above-mentionedevaluation items in the same manner as in Example 1. The results ofmeasurements are shown in Table 4.

Comparative Example 4

A sintered material was prepared in the same manner as in Example 1 inthe same manner as in Example 1 except that the addition amount of aid(strontium carbonate) was changed to 3% by mass and that “Thesilicon-silicon carbide porous material of honeycomb structure wasmaintained at normal pressure (atmospheric pressure) in a nitrogenatmosphere at 1,450° C. for 4 hours and further at 1,750° C. for 4 hours(during-nitriding firing).” was changed to “The silicon-silicon carbideporous material of honeycomb structure was maintained at normal pressure(atmospheric pressure) in a nitrogen atmosphere at 1,100° C. for 12hours.” In the same manner as in Example 1, test pieces were cut outfrom the sintered material and measured for the above-mentionedevaluation items. The results of measurements are shown in Table 4.TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Aid (SrCO₃) 1 1 1 0 1 (mass %)Pore former 10 10 0 10 10 (starch) (mass %) Before-nitriding Yes Yes YesYes Yes firing of formed body Firing Pressure 1.3 Pa 1.3 Pa 1.3 Pa 1.3Pa Normal conditions Atmosphere Ar Ar Ar Ar Ar (oxygen of formed partialbody pressure: 10 Pa) Temp. 1450° C. 1450° C. 1450° C. 1450° C. 1450° C.Time 2 hr 2 hr 2 hr 2 hr 2 hr During- Pressure Normal Normal NormalNormal Normal nitriding Atmosphere N₂ N₂ N₂ N₂ N₂ firing Temp. and 1450°C. & 1450° C. & 1450° C. & 1450° C. & 1450° C. & conditions time 4 hr 4hr 4 hr 4 hr 4 hr then then then then then 1750° C. & 1750° C. & 1750°C. & 1750° C. & 1750° C. 4 hr 4 hr 4 hr 4 hr & 4 hr Before-nitridingfiring Separate Same Separate Separate Separate and during-nitridingfirings firing firings firings firings firing (temp. of (room (1400° C.)(room (room (room temp.) atmosphere change) temp.) temp.) temp.) Ex. 6Ex. 7 Ex. 8 Ex. 9 Ex. 10 Aid (SrCO₃) 1 1 1 1 1 (mass %) Pore former 1030 40 0 10 (starch) (mass %) Before-nitriding Yes Yes Yes Yes Yes firingof formed body Firing Pressure 1.3 Pa 1.3 Pa 1.3 Pa 1.3 Pa 1.3 Paconditions Atmosphere Ar Ar Ar Ar Ar of formed body Temp. 1450° C. 1450°C. 1450° C. 1450° C. 1450° C. Time 2 hr 2 hr 2 hr 2 hr 2 hr During-Pressure Normal Normal Normal Normal Normal nitriding Atmosphere N₂ N₂N₂ N₂ N₂ firing Temp. and 1450° C. & 1450° C. & 1450° C. & 1450° C. &1450° C. & conditions time 4 hr 4 hr 4 hr 4 hr 4 hr then then then thenthen 1750° C. & 1750° C. & 1750° C. & 1750° C. & 1750° C. & 4 hr 4 hr 4hr 4 hr 4 hr Before-nitriding firing Separate Separate Separate SeparateSeparate and during-nitriding firings firings firings firings firingsfiring (temp. of (room (room (room (room (room atmosphere change) temp.)temp.) temp.) temp.) temp.) Ex. 11 Ex. 12 Ex. 13 Ex. 14 Aid (SrCO₃) 1 31 1 (mass %) Pore former 30 10 10 10 (starch) (mass %) Before-nitridingYes Yes Yes Yes firing of formed body Firing Pressure 1.3 Pa 1.3 PaNormal Normal conditions Atmosphere Ar Ar Ar (oxygen Ar (oxygen offormed partial partial body pressure: 10 Pa) pressure: 10 Pa) Temp.1450° C. 1450° C. 1450° C. 1450° C. Time 2 hr 2 hr 2 hr 2 hr During-Pressure Normal Normal Normal Normal nitriding Atmosphere N₂ N₂ N₂ N₂firing containing 5 containing 5 conditions vol. % of vol. % of hydrogenhydrogen Temp. and 1450° C. & 1200° C. & 1450° C. & 1450° C. & time 4 hr12 hr 4 hr 4 hr then 1750° C. & 4 hr Before-nitriding firing SeparateSeparate Separate Same firing and during-nitriding firings firingsfirings (1450° C.) firing (temp. of (room (room (room temp.) atmospherechange) temp.) temp.)

TABLE 2 Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Aid (SrCO₃)  1 1  1  3 (mass %) Pore former 10 10 10 10 (starch) (mass %)Before-nitriding No Yes Yes Yes firing of formed body Firing PressureNormal Normal 1.3 Pa conditions Atmosphere Ar Ar Ar of formed (oxygen(oxygen body partial partial pressure: pressure: 100 Pa) 100 Pa) Temp.1450° C. 1450° C. 1450° C. Time 2 hr 2 hr 2 hr During- Pressure NormalNormal Normal Normal nitriding Atmosphere N₂ N₂ N₂ N₂ firing Temp. andtime 1450° C. & 1450° C. & 1450° C. & 1100° C. & conditions 4 hr 4 hr 4hr 12 hr then then then 1750° C. & 1750° C. & 1750° C. & 4 hr 4 hr 4 hrBefore-nitriding firing Not Separate Separate Separate andduring-nitriding sintered firings (room firings firings firing (temp. ofand collapsed temp.) (room (room atmosphere change) upon touch. temp.)temp.)

TABLE 3 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10Open porosity (%) 60 60 47 61 59 60 75 78 34 60 Average pore diameter 2726 11 24 26 8 10 32 3 53 (μm) Specific surface area 0.3 0.3 0.3 0.4 0.30.5 0.4 0.4 0.3 0.4 (m²/g) Four-point bending strength 16 16 27 15 14 810 1 53 2 (MPa) Gas permeability 2.0 1.9 1.0 2.1 1.9 1.2 1.9 18 0.1 15coefficient (μm²) Condition 1 Suitable Suitable Suitable SuitableSuitable Suitable Suitable Suitable Suitable Suitable 1200° C. × 24 hrtreatment 4.4 4.4 2.6 4.4 4.3 4.5 5.0 5.0 2.4 5.0 (mass %) Heatresistance temp. 1300 1300 1400 1300 1300 1300 1200 1100 1400 1200 (°C.) Ex. 11 Ex. 12 Ex. 13 Ex. 14 Open porosity (%) 75 60 50 50 Averagepore diameter 3 22 15 19 (μm) Specific surface area 0.9 0.4 0.5 0.3(m²/g) Four-point bending strength 21 18 20 15 (MPa) Gas permeability0.2 1.5 2.2 4.1 coefficient (μm²) Condition 1 Suitable Suitable SuitableSuitable 1200° C. × 24 hr treatment 5.5 4.5 4.4 4.3 (mass %) Heatresistance temp. 1150 1300 1400 1400 (° C.)

TABLE 4 Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Open porosity(%) Not sintered 60 60 60 and collapsed upon touch. Average porediameter — 8 5 25 (μm) Specific surface area — 3.1 1.2 0.3 (m²/g)Four-point bending — 11 21 14 strength (MPa) Gas permeability — 0.1 0.12.0 coefficient (μm²) Condition 1 — Unsuitable Suitable No siliconnitride formed and unsuitable 1200° C. × 24 hr — 6.2 6.1 6.5 treatment(mass %) Heat resistance temp. — 1100 1100 1100 (° C.)

As is clear from Tables 3 and 4, the porous materials of Examples 1 to14 according to the present invention are low in the generation amountof columnar silicon nitride of given size and have a specific surfacearea of pores, of 1 m²/g or less; therefore, these porous materials aresuperior in heat resistance and gas permeability. Having an openporosity of 40 to 75%, they are clearly superior in strength and gaspermeability. Further, having an average pore diameter of 5 to 50 μm,they are greatly superior in strength and gas permeability. The porousmaterial of Comparative Example 4 is large in the amount of mass changein a 1,200° C.×24 hr treatment and inferior in heat resistance. This isbecause the during-nitriding firing is conducted at a low temperature(1,100° C.) and resultantly the formation of silicon nitride is notenough and silicon remains unnitrided in a large mount.

INDUSTRIAL APPLICABILITY

As described above, in the porous material of the present invention,silicon carbide particles as an aggregate are bonded with one anothervia silicon nitride as a binder; therefore, the present porous materialis superior in heat resistance. Further, in the porous material, nocolumnar silicon nitride (silicon nitride whisker) is present on thesurface of the silicon nitride within each pore, or, even when siliconnitride whiskers are inevitably present there, the number of thecolumnar silicon nitride having a thickness of more than 2 μm and anaspect ratio of less than 10 is greater than that of the columnarsilicon nitride having a thickness of 2 μm or less or an aspect ratio of10 or more; therefore, part of the pores is not blocked by the siliconnitride whiskers, making the present porous material superior in gaspermeability.

1-11. (canceled)
 12. A porous material wherein silicon carbide particlesas an aggregate are bonded with one another via silicon nitride as abinder in such a state that pores are present between the siliconcarbide particles, wherein no columnar silicon nitride (silicon nitridewhisker) is formed on the surface of the silicon nitride within eachpore, or that, even when columnar silicon nitride is inevitably formedthere, the number of the columnar silicon nitride having a thickness ofmore than 2 μm and an aspect ratio of less than 10 is greater than thatof the columnar silicon nitride having a thickness of 2 μm or less or anaspect ratio of 10 or more.
 13. A porous material wherein siliconcarbide particles as an aggregate are bonded with one another viasilicon nitride as a binder in such a state that pores are presentbetween the silicon carbide particles, wherein the pores have a specificsurface area of 1 m²/g or less.
 14. A porous material according to claim12, wherein an open porosity is 40 to 75%.
 15. A porous materialaccording to claim 13, wherein an open porosity is 40 to 75%.
 16. Aporous material according to claim 12, wherein the pores have an averagepore diameter of 5 to 50 μm.
 17. A porous material according to claim13, wherein the pores have an average pore diameter of 5 to 50 μm.
 18. Aporous material according to claim 12, which has a heat resistancetemperature of 1,200° C. or more.
 19. A porous material according toclaim 13, which has a heat resistance temperature of 1,200° C. or more.20. A porous material according to claim 12, which has a gaspermeability coefficient of 1 μm² or more.
 21. A porous materialaccording to claim 13, which has a gas permeability coefficient of 1 μm²or more.
 22. A method for producing a porous material wherein siliconcarbide particles as an aggregate are bonded with one another viasilicon nitride as a binder in such a state that pores are presentbetween the silicon carbide particles, wherein no columnar siliconnitride (silicon nitride whisker) is formed on the surface of thesilicon nitride within each pore, or that, even when columnar siliconnitride is inevitably formed there, the number of the columnar siliconnitride having a thickness of more than 2 μm and an aspect ratio of lessthan 10 is greater than that of the columnar silicon nitride having athickness of 2 μm or less or an aspect ratio of 10 or more, wherein themethod comprises the steps of: mixing at least silica, silicon nitrideand a pore former; firing the resulting mixture at 1,400 to 1,500° C. inan inert gas atmosphere or reduced-pressure atmosphere where the oxygenpartial pressure is 10 Pa or less to prepare a silicon-silicon carbideporous material; and nitriding and firing the silicon-silicon carbideporous material at 1,200 to 1,800° C. in a nitrogen atmosphere.
 23. Amethod for producing a porous material wherein silicon carbide particlesas an aggregate are bonded with one another via silicon nitride as abinder in such a state that pores are present between the siliconcarbide particles, wherein the pores have a specific surface area of 1m²/g or less, wherein the method comprises the steps of: mixing at leastsilica, silicon nitride and a pore former; firing the resulting mixtureat 1,400 to 1,500° C. in an inert gas atmosphere or reduced-pressureatmosphere where the oxygen partial pressure is 10 Pa or less to preparea silicon-silicon carbide porous material; and nitriding and firing thesilicon-silicon carbide porous material at 1,200 to 1,800° C. in anitrogen atmosphere.
 24. A method for producing a porous materialaccording to claim 11, wherein, after preparing the silicon-siliconcarbide porous material, the atmosphere used therein is changed to anitrogen atmosphere without lowering the temperature to room temperatureand keeping the temperature at 1,200° C. or more, and nitriding andfiring the silicon-silicon carbide porous material at 1,200 to 1,800° C.in the nitrogen atmosphere is conducted.
 25. A method for producing aporous material according to claim 12, wherein, after preparing thesilicon-silicon carbide porous material, the atmosphere used therein ischanged to a nitrogen atmosphere without lowering the temperature toroom temperature and keeping the temperature at 1,200° C. or more, andnitriding and firing the silicon-silicon carbide porous material at1,200 to 1,800° C. in the nitrogen atmosphere is conducted.
 26. A methodfor producing a porous material according to claim 22, wherein, afterpreparing the silicon-silicon carbide porous material, nitriding andfiring the silicon-silicon carbide porous material at 1,200 to 1,800° C.is conducted in a nitrogen atmosphere containing 0.1% by volume or moreof hydrogen.
 27. A method for producing a porous material according toclaim 23, wherein, after preparing the silicon-silicon carbide porousmaterial, nitriding and firing the silicon-silicon carbide porousmaterial at 1,200 to 1,800° C. is conducted in a nitrogen atmospherecontaining 0.1% by volume or more of hydrogen.
 28. A method forproducing a porous material according to claim 22, wherein, after thepreparation of the silicon-silicon carbide porous material, theatmosphere is changed to a nitrogen atmosphere containing 0.1% by volumeor more of hydrogen (a hydrogen-containing nitrogen atmosphere) withoutlowering the temperature to room temperature and keeping the temperatureat 1,200° C. or more, and nitriding and firing the silicon-siliconcarbide porous material at 1,200 to 1,800° C. in the hydrogen-containingnitrogen atmosphere is conducted.
 29. A method for producing a porousmaterial according to claim 23, wherein, after the preparation of thesilicon-silicon carbide porous material, the atmosphere is changed to anitrogen atmosphere containing 0.1% by volume or more of hydrogen (ahydrogen-containing nitrogen atmosphere) without lowering thetemperature to room temperature and keeping the temperature at 1,200° C.or more, and nitriding and firing the silicon-silicon carbide porousmaterial at 1,200 to 1,800° C. in the hydrogen-containing nitrogenatmosphere is conducted.
 30. A honeycomb structure constituted by aporous material wherein silicon carbide particles as an aggregate arebonded with one another via silicon nitride as a binder in such a statethat pores are present between the silicon carbide particles, wherein nocolumnar silicon nitride (silicon nitride whisker) is formed on thesurface of the silicon nitride within each pore, or that, even whencolumnar silicon nitride is inevitably formed there, the number of thecolumnar silicon nitride having a thickness of more than 2 μm and anaspect ratio of less than 10 is greater than that of the columnarsilicon nitride having a thickness of 2 μm or less or an aspect ratio of10 or more.
 31. A honeycomb structure constituted by a porous materialwherein silicon carbide particles as an aggregate are bonded with oneanother via silicon nitride as a binder in such a state that pores arepresent between the silicon carbide particles, wherein the pores have aspecific surface area of 1 m²/g or less.